1. Field of Endeavor
The invention relates generally to safe storage of compounds, in particular volatile compounds. In one aspect, the present disclosure relates to the use of foam-cell material to store volatile compounds. In another aspect, the present disclosure provides for the safe storage of a source from which hydrogen may be extracted.
2. Description of Related Art
Recent improvements in the design and manufacture of hydrogen/air fuel cells have increased interest in the use of fuel cells as a replacement for batteries and other power supplies (e.g., vehicle engines). Because hydrogen/air fuel cells can operate on very energy-dense fuels and are quiet and efficient, fuel-cell-based power supplies are considered a promising future power source. Many fuel cells operate using hydrogen gas for fuel, and oxygen (typically from air) as an oxidant. Unfortunately, reliable, convenient, and compact hydrogen sources do not yet exist, so fuel cells have yet to receive widespread commercial or military use. Fuel cells, however, represent relatively mature technology and are commercially available.
As discussed above, there is a great need and interest in hydrogen generators, including compact hydrogen generators, that extract hydrogen gas from a source. Examples of such sources/volatiles from which hydrogen may be obtained include, but are not limited to, butane, propane and anhydrous ammonia. As can be appreciated, use of hydrogen generators for providing hydrogen gas will likely be restricted unless the sources of hydrogen can be stored in a safe manner. For example, if anhydrous ammonia is the hydrogen source, a method and apparatus for storing ammonia must reduce and/or minimize the potential for a dangerous ammonia release if the integrity of the storage apparatus is compromised. Successful development of a safe ammonia-storage tank is beneficial because it facilitates rapid deployment of ammonia-based hydrogen sources for compact fuel-cell power supplies.
Several approaches are available for hydrogen generation and/or storage. These include hydrocarbon and methanol fuel reforming, hydrogen absorption into metal hydrides, hydrogen-generating chemical reactions, and ammonia decomposition (Blomen, L., and M. N. Mugerwa. 1993. Fuel Cell Systems. Plenum Press, New York; Bloomfield, D. P., V. J. Bloomfield, P. D. Grosjean, and J. W. Kelland. 1995. Mobile Electric Power. Analytic Power Corp., NTIS Report ADA296709). Adsorbent-based approaches offer reduced storage pressure, but often at a cost of vastly increased storage volume and mass.
Ammonia decomposition and ammonia-based chemical reactions are attractive methods for hydrogen generation because the required chemical reactors tend to be relatively small, simple, and easy to control. Ammonia decomposition has received relatively little attention, however, because of ammonia's toxicity and foul odor, and because it is generally not economical for power production except in remote, low-power applications (Appleby, A. J., and F. R. Foulkes. 1989. Fuel Gel/Handbook Van Nostrand Reinhold, New York). In spite of these drawbacks, hydrogen from ammonia is attractive for at least two reasons: (I) The usable hydrogen per kilogram of fuel is relatively high; and (2) ammonia-based fuel-cell systems can be deployed much sooner than the more complicated hydrocarbon-based fuel reformers.
Before ammonia-based hydrogen generators will gain acceptance, the problem of safe ammonia storage must be addressed. Ammonia is a toxic gas that can rapidly damage the eyes and respiratory tract upon exposure to concentrations in the range of about 500-1000 ppm. Exposure to higher concentrations (>5000 ppm) even for short periods can lead to respiratory failure and death (Nielsen, A. 1995. Ammonia: catalysis and Manufacture. Springer-Verlag. London). An ammonia-based hydrogen generator operating in an enclosed environment must not have the potential for rapid ammonia release, as this may be harmful or even deadly for surrounding personnel.
The ammonia-based hydrogen generators currently under development (e.g., Powell, M R. M S Fountain, C J Call, A S Chellappa. 2002. “Ammonia-Based Hydrogen Generation for Fuel Cell Power Supplies.” Army Science Conference 2002, Orlando, Fla. Dec. 2-5, 2002) employ lightweight storage tanks made from either aluminum or titanium. These tanks have a mass of approximately 120 g and an ammonia storage volume of about 0.7 liters. The tanks are designed to withstand >1000 psig to ensure they do not burst in response to ammonia vapor pressure, which can exceed 250 psi at temperatures greater than 40° C. However, the storage tanks are not designed to withstand punctures from sharp objects or projectiles such as bullets. Further, there is the possibility for failure of tubing and/or reactor components downstream of the ammonia-storage tank, all of which could result in rapid release of ammonia. Before ammonia-based hydrogen sources can be widely marketed, the ammonia storage tanks must be improved to guard against rapid ammonia release in the event of tank puncture.
Currently, safe storage of ammonia (and other selected hazardous liquefied gases) requires use of relatively heavy, thick-walled tanks or loading the ammonia onto high-capacity adsorbents. The storage units in both approaches are heavy and result in undesirable increases in mass of end-use systems such as hydrogen generators for fuel cells. For example, an ammonia-storage system with a 500-gram capacity will have a total mass of about 2000 g or more (capacity<20 wt.-%) if a standard storage tank is used. If an adsorbent is used instead, the mass of adsorbent is expected to be at least three times the mass of ammonia stored, so the resulting storage system will have a mass greater than 2000 g (capacity<20 wt.-%).
Monolithic storage structures for gases have received relatively little attention in the literature. This is largely because there is not a perceived need for the ability to store small quantities of toxic or flammable gases under pressure in a small volume. Propane and butane are sold commercially in small quantities as liquids, but safety concerns are mitigated through the use of a heavy storage vessel and warnings regarding indoor storage and use of the fuel. As compact, lightweight fuel-cell power systems become more prevalent, however, greater emphasis is expected on the need to safely store small quantities of these materials for indoor use.
Some work along these lines has been performed at the Oak Ridge National Laboratory as part of a program to develop passenger vehicles that can run on natural gas. Storage of the natural gas is the principal obstacle to these vehicles because natural gas cannot be liquefied under ambient temperature conditions. Burchell and Rogers (2000) (Burchell, T. and M. Rogers. 2000. “Low Pressure Storage of Natural Gas for Vehicular Applications.” SAE Technical Paper Series. 2000-01-2205. SAE, Warrendale, Pa.) report on a monolithic storage structure utilizing adsorption of natural gas. Adsorbent fibers are configured into a monolithic block with high adsorption capacity and high thermal conductivity, which are both desirable properties for the vehicle application (high thermal conductivity allows rapid filling of the adsorbent without overheating).
In particular, this prior art approach, however, is not likely to be of use for safe ammonia storage. Adsorbent-based approaches suffer from relatively low ammonia storage density and the need to provide heat to desorb the ammonia from the adsorbent. Further, this adsorbent monolith has a high thermal conductivity, which is counter-productive for safe ammonia storage. Low thermal conductivity of the storage matrix is preferred to help retard vaporization of volatiles, for example ammonia, from the monolith. If heat cannot quickly reach the vapor-liquid interface, volatilization of ammonia will be slowed.